Pre-coated metals generated in continuous coil coating lines processing rolled stock such as cold rolled steel, hot dip galvanized steel, stainless steel, tin plated steel and aluminum, represent a major industrial activity today. Steel, excluding hot rolled steel, is increasingly being coated in this manner. The organic products normally employed for this purpose are based on silicones, polyesters, epoxies, urethanes, acrylics and combinations thereof, PVC plastisols and fluorocarbons. Most of these products are solvent based i.e. contain a volatile organic solvent, though some are also available as powdered resin to be applied as powder coatings. Invariably these products require their specific primers as these lack the ability to adhere directly to substrates in the short oven dwell times of continuous coil coating lines.
Self-priming coating formulations have been generally made based on alkyds (U.S. Pat. No. 5,539,032) polyurethanes (U.S. Pat. Nos. 5,427,821, 5,403,880 and 5,236,983), epoxies (U.S. Pat. Nos. 5,491,185, 5,202,367 and 5,130,361), phenoxy (U.S. Pat. No. 5,274,021) and other resins and combinations thereof. The aforementioned formulations, however, are meant for structural applications with long curing cycles where they replace conventional primer plus top coats. Such compositions are not suited to the intricate and rigorous demands of the continuous coil coating lines, where coatings ought to harden/cure on the substrate in less than a minute and should be extremely flexible and scratch resistant so as to withstand post forming rigors in engineering, appliance and construction industries. In addition to these properties the coatings should be chemically resistant and possess good weatherability and aesthetic appeal to function as a top coat.
The products used for metal pre-coating in continuous coil coating lines ranging from vinyl plastisols and fluorocarbons to thermosetting resins as stated in prior art have a limitation in that these do not generally offer a combination of high scratch hardness with good flexibility. Hardness of the thermosetting resinous coating depends on the density of cross links of these resins on cure. As this density increases, the material hardens but begins to lose its flexibility. Contrarily, a coating that is inherently flexible such as plastisol, has a low Glass Transition Temperature (Tg) and hardness. Further, these coatings are also poor in thermal resistance. Therefore such coatings are unsuitable as abrasion resistant coatings and in applications requiring high heat resistance. On the other hand, coatings with high surface hardness normally contain large amounts of inorganic pigments and additives, mainly zinc and its compounds. Coatings rich in inorganic content generally demonstrate poor flexibility restricting their use in applications requiring the ability of coated substrates to withstand forming, drawing and other related operations.
Another general limitation indicated in prior art is the requirement of selective primers for the presently used coatings. The primers ordinarily contain a large percentage of inorganic additives and corrosion inhibiting pigments. Such additives in some cases include chromate based compounds which are considered to be toxic and harmful. These primers are normally used to provide an anchor for the top coat. Priming of metal substrate is expensive and time consuming, as this adds another operation to the coating scheme besides increasing the cost of removal of the paint during stripping operation. It also adds to the overall thickness of the coating, which may be undesirable in certain applications.
Self-priming products offer very high corrosion resistance if they have good barrier characteristics. A high dielectric strength/volume resistivity is vital for withstanding electrochemical corrosion on a variety of substrates more so on ferrous surfaces. Adhesion to the substrate following the extremely rapid curing process adopted by continuous coil coating lines is essential.
Self-priming coatings so far proposed for metallic structures as disclosed in the U.S. Patents referred to above have long cure cycles. No satisfactory self-priming coating system is available at present for ferrous/non-ferrous substrate that is amenable to continuous coil coating applications and possesses the combination of high surface hardness and good flexibility with corrosion resistance and weatherability.
Accordingly, there was a long felt need to provide a coating composition, which ensures hardness with flexibility in a thin self-priming coat. The inventor of the instant invention has found that the combination of a vinyl terpolymer with an alkyd resin which cross links through hydroxyl groups exhibits a synergistic effect to produce the much desired coating having good flexibility with high surface hardness while ensuring quick curing for continuous coil coating application at the same time. It will be obvious to someone skilled in the art that such properties offer important advantages in a variety of applications.